In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
The term “nano-structured” or “nanostructure” material is used by those familiar with the art to designate materials including nanoparticles with a particle size or less than 100 nm, such as nanotubes (e.g.—carbon nanotubes). These types of materials have been shown to exhibit certain properties that have raised interest in a variety of applications.
Current x-ray tubes comprise a target with a single (or dual) focal point(s) (where electrons bombard a target and x-rays are emitted), and either a single cathode or dual cathodes. Electrons are generally thermionically emitted from the cathode(s) and are focused to impact a small area on the target. For imaging or diagnostic purposes over a large area, either the object or the x-ray tube (or both) has to be moved over the area.
One example of such a use for x-rays includes computed tomography (CT). FIGS. 1 (a-d) depict the general arrangement of a CT scanner 100 in which an x-ray source 102 (typically an x-ray tube) is rotated (direction indicated by the arrow) about the object to be imaged 104. The data can be collected by either a single detector 106 coordinately moving with the x-ray source 102 (see FIG. 1a), multiple detectors 106 coordinately moving with the x-ray source 102 (see FIGS. 1b and 1c), or multiple stationary detectors 106 (see FIG. 1d). As a consequence of the current CT designs, the scanners are usually large with complicated mechanical systems. Scanners can be limited by the rotation speed of the x-ray tube and by the size and weight of the apparatus. This last limitation can adversely affect the capture of clear images of moving parts, such as a beating heart.
Dynamic three-dimensions (3D) images can be acquired by so-called “fifth generation CT scanners.” An example of a 5th generation CT scanner 200 is shown in FIG. 2. An electron beam 202 is produced by a stationary electron gun 204 in a vacuum system 206. The electron beam 202 is focused by a focus coil 208 and scanned by a deflection coil 210 such that the applied magnetic field positions the electron beam 202 across any one of several stationary metal target rings 212 from which an x-ray is emitted. The emitted x-rays 214 project toward the object 216 to be imaged and are detected by a stationary detector 218 and processed by a data acquisition system 220. Although such a system is capable of producing dynamic 3D images, the scanner 200 is very large and costly to fabricate.
In addition to the design limitations of current CT scanners, current x-ray sources use heated metal filaments as the sources of electrons. Because of the thermionic emission mechanism, a very high operating temperature is required, typically in the order of 1000-1500° C. The high operating temperatures results in problems such as short lifetime of the filament, slow response time (i.e., time to warm up the filament before emission), high-energy consumption, and large device size.
Further, because of the fundamental physics of the thermionic emission mechanism, electrons are emitted in all directions. In x-ray tube applications, an additional electrical field can be applied to bring the electrons into focus at the target, but with attendant complexity and cost. Additionally, such conventional techniques of focusing electron beams have certain disadvantages, such as limits on the uniformity and size of the focusing spots.
Carbon nanotube materials have been shown to be excellent electron field emission materials. In this regard, carbon nanotube materials have been shown to possess low electron emission threshold applied field values, as well as high emitted electron current density capabilities, especially when compared with other conventional electron emission materials.
Commonly owned U.S. Pat. No. 6,630,772 (Ser. No. 09/296,572 entitled “Device Comprising Carbon Nanotube Field Emitter Structure and Process for Forming Device”) the disclosure of which is incorporated herein by reference, in its entirety, discloses a carbon nanotube-based electron emitter structure.
Commonly owned U.S. Pat. No. 6,552,380 (Ser. No. 09/351,537 entitled “Device Comprising Thin Film Carbon Nanotube Electron Field Emitter Structure”), the disclosure of which is incorporated herein by reference, in its entirety, discloses a carbon-nanotube field emitter structure having a high emitted current density.
Commonly owned U.S. Pat. No. 6,277,318 entitled “Method for Fabrication of Patterned Carbon Nanotube Films,” the disclosure of which is incorporated herein by reference, in its entirety, discloses a method of fabricating adherent, patterned carbon nanotube films onto a substrate.
Commonly owned U.S. Pat. No. 6,553,096 (Ser. No. 09/679,303 entitled “X-Ray Generating Mechanism Using Electron Field Emission Cathode”), the disclosure of which is incorporated herein by reference, in its entirety, discloses an X-ray generating device incorporating a nanostructure-containing material.
Thus, there is a need in the art to address the above-mentioned disadvantages associated with methods and devices for generating x-rays and for applying x-rays in applications in imaging environments, such as computed tomography. Further, there is a need in the art for imaging solutions in both static and dynamic environments.